This invention relates to the improvement of event detection capabilities for cable monitoring devices utilizing time domain reflectometry principles as installed in geotechnical materials.
Time Domain Reflectometry, or TDR, is a remote sensing electrical measurement technique that has been used for many years to determine the spatial location and nature of various objects. An early form of TDR, dating from the 1930s, that most people are familiar with is radar. The type of TDR most commonly referred to by the acronym in the industry is coaxial TDR. Coaxial TDR is essentially a xe2x80x9cclosed circuit radarxe2x80x9d. It involves sending an electrical pulse along a coaxial cable and using an oscilloscope to observe the echoes returning back to the input. This technique was reported in the literature in the 1930""s and 40""s for testing telephone coaxial cables. Numerous TDR articles and books have been written on the subject since.
Optical TDR functions by sending a light pulse down a fiber optic cable. This pulse is reflected by broken ends or attenuated by cable bends. Sonar, also a type of TDR, operates by pulsing a sound wave through a media and examining the returned echo for reflection and attenuation compared to the original signal.
TDR has been actively investigated by both government and private enterprise for uses in monitoring mining induced displacements in the geologic mass surrounding the mine. U.S. Bureau of Mine research was started in the 1960""s, when TDR was primarily used to locate breaks in electrical power cables. Since then, use of the method has expanded, but has still not reached its full potential in the field.
If a geologic material is subject to excess stress, whether generated by natural events (excess rainfall, earthquakes, etc.), or by human events (excavation) it displaces to equilibrate this excess stress. This will result in discrete displacement (failure along a plane) or distributed displacement within the geologic mass. TDR cable monitoring is generally conducted by placing a TDR capable cable in a drill hole in the geologic mass. Prior to installation, the cable may be crimped to provide reference reflections in the cable at known physical locations in the rock mass. After crimping, the cable is inserted into a borehole, and bonded to the surrounding rock with a cement grout. At locations where progressive geologic movement is sufficient to fracture the grout, cable deformation occurs that can be monitored with a TDR cable tester.
This technique has been tested at Syncrude in Canada, amongst other locations. Syncrude operates an oil sand mine in northern Alberta, Canada. The oil sand is mined by large draglines, which operate adjacent to the edge of a highwall that varies in height from 40 to 60 m. Coaxial cables were installed in vertical holes at three highwall locations in the immediate vicinity (less than 10 m) of slope inclinometers so that a comparison could be made between the two types of instrumentation. The objective of these installations was to assess the ease or difficulty of installation, suitability to field conditions, ease or difficulty of data acquisition, comparison with existing monitoring procedures, and sensitivity of TDR to slope movements.
In addition to the field study, an extensive laboratory test program was implemented to correlate TDR reflection magnitude with shear deformation of grouted cables.
It was concluded that TDR represented a promising technology for slope monitoring, but modifications would be required to increase its sensitivity in oil sands and stiff clay soils. Applications in hard rock mining, such as block caving, indicate that block displacement is sufficiently discrete to shear the cable at distinct points, giving a better response than would be expected in stiff clays. In addition, proper selection of the type of cable to be encapsulated, as well as the encapsulation material, such as stiffer grout, can be utilized to increase the system sensitivity to displacement.
It is further known that xe2x80x9ccrimpsxe2x80x9d or buttons can be placed on the monitoring cable prior to installation. These impart a manufactured defect in the cable that serves as a reference point for any adjacent TDR reflections. These crimps are not anchors and serve exclusively as reference points for TDR measurement.
In addition, the art in the field of TDR geotechnical monitoring has become almost exclusively focused on the magnitude and type of displacement within the geologic material being monitored. The invention described herein focuses, alternatively, on maximizing the probability of detecting any significant displacement within the geologic material. While the former may be of significance for exact definition of a known feature, or precisely determining the type of failure that may be occurring, within the geologic medium, it lacks sensitivity regarding the specific questions xe2x80x9cIs displacement occurring in the geologic material being monitored?xe2x80x9d and xe2x80x9cIf so, where, approximately, is the displacement occurring?xe2x80x9d These two questions are of utmost criticality in determining further action and monitoring in any geotechnical monitoring situation. If a slope containing a gas pipeline is failing, a simple, effective monitoring system should indicate immediately that action is required based on the fact that some event is occurring in the geologic material composing the slope, a fact that the proposed invention capitalizes on. The invention proposed herein increases the sensitivity of the geotechnical monitoring system such that it can answer such questions quickly, with more sensitivity, more inexpensively, and with less technical input than the monitoring systems utilized to date.
Little attempt has been made to address the shortcomings of TDR applications mentioned above from the Syncrude report, i.e. modifications are necessary to increase TDR sensitivity in oil sands (sands) and stiff clay soils. The response has generally been to install more sensitive cables utilizing the same old techniques. Thus, TDR monitoring is under-utilized in areas composed of such materials.
The prior art, in terms of sensor cables and cable anchor devices, suffers from a number of flaws:
(a) anchor devices for cables of the prior art are designed for attaching cables to a surface for transmission purposes. As such they are not designed for, and are not applicable, to restricting cable motion in a three-dimensional solid for monitoring purposes.
(b) anchor devices for cables of the prior art are not designed for being encapsulated in a surrounding media such as grout, which is a requirement for geotechnical monitoring.
(c) anchor devices for cables of the prior art are not designed to function in terms of displacement amplification at the end of oblong anchors due to rotation about the anchor""s centroid.
(d) with minor exception, the concept of an anchor system being attached to a sensor cable for point strain monitoring, event detection, or continuous strain monitors has been totally ignored.
(e) prior art anchor devices are all too expensive and unnecessarily cumbersome for geotechnical use in light of the number of such devices required for a single sensor cable installation.
(f) most of the existing sensor cable arrangements attempt to quantify the magnitude of displacement between two pre-determined, fixed points along the sensor cable length. This would be unrealistic for geotechnical monitoring as the actual location of a failure surface, i.e., a point to be monitored, is unknown upon installation of the sensor in undisturbed material. For initial analysis, it is the occurrence and approximate location of the failure (deformation) that is critical to the success of geotechnical monitoring, not the absolute deformation magnitude.
(g) the sensor cable devices of the prior art require a specific loading mode. This is generally induced by tension or compression (buckling). Such limitations are too constraining and unnecessary for an event detecting geotechnical monitoring system. If such absolute constraints are placed on a geotechnical monitoring system it would be extremely difficult to utilize given the complete range and combination of tension, compression, and shear found in any loading situation within a three-dimensional solid.
(i) most of the existing sensor cables seek to obtain a highly reproducible loss within the cable as a function of deformation. This is a limiting factor for geotechnical event detection. Any change within the properties of the sensor cable indicates motion of the surrounding medium and is to be detected and observed. So long as the change is detectable and provides an indication of displacement within the monitored material, the magnitude of the change is relatively unimportant.
The prior art event detectors are also limited by surficial applications, being applied to the surface of a structure in order to detect a specific physical happening. This is of little practicality for a geologic event that will occur somewhere three-dimensionally within a geologic medium. These prior art devices make no mention of an encapsulating media. As event detectors, the prior art utilizes an interlocking tooth arrangement to create microbends in a fiber optic cable if a specific xe2x80x9ceventxe2x80x9d occurs. This does not amplify displacement detected in a surrounding geologic environment. Nor is it particularly sensitive to omni-directional motion, being more specifically designed for motion oriented from a specific direction.
Thus, objects of the present invention are:
(a) to provide a sensor cable composed of material capable of conducting light, sound, or electricity to be inserted by appropriate means into a material to be examined, allowing selection of the most appropriate TDR method (optical, coaxial, sonar, or other) for the specific mode of deformation expected within the geologic mass;
(b) to provide a sensor cable installed three-dimensionally and internally within the body to be examined;
(c) to provide a cable anchor that restricts longitudinal movement of the cable within the encapsulating medium, whether it be soil, grout, chemical or other agent;
(d) to provide a cable anchor that, when displaced, acts as a displacement amplifier, undergoing translation/rotation within the encapsulating agent);
(e) to provide a cable anchor sufficiently rigid and strong such as to promote bearing failure in the encapsulating material upon translation/rotation by geologic material displacement, if such behavior is desired;
(f) to provide a cable anchor that can be engineered as to length, shape, and material properties such as to specifically determine at what loads and deflections bearing failure will take place in the encapsulating medium as a function of cable anchor rotation;
(g) to provide a monitoring system such that sensor cable strength and modulus can be chosen such as to impact on the relative motion of the adjacent cable anchors;
(h) to provide a monitoring system such that the material properties of the of the encapsulating agent may be chosen such as to determine the stress levels of bearing failure around the cable anchor upon geologic material displacement;
(i) to provide a geotechnical monitoring system that may be installed without a borehole in a geologic mass, but instead be inserted in the mass by utilization of fluid jet action;
(j) to provide a geotechnical monitoring device utilizing TDR principles functioning as an event detector independent of the shape the cable assumes upon deformation of both itself and surrounding media;
(k) to provide a geotechnical monitoring device utilizing TDR principles functioning as an event detector independent of the stress type (shear, tensile, or compression) the sensing cable experiences during deformation of both itself and surrounding media;
(l) to provide a geotechnical monitoring device capable of event detection along its design encapsulated length such that deformation in the surrounding geologic medium may be located proximal to the cable disturbance between anchors;
(m) to provide access through an actively failing and displacing geologic body such that a sensor cable can be placed and utilized for monitoring at and below the assumed failure mass""s base plane;
(n) to substantially improve detection capability for cable based TDR systems in soils and weak geologic rock formations; and
(o) to provide improved performance for cable based TDR systems in hard rock environments as a function of anchor, encapsulating agent, and cable selection.
Further, the proposed invention will improve upon present TDR techniques, utilizing increasing cable sensitivity by amplifying displacement within the geologic mass. Thus, if very sensitive instruments and cables, as presently used, were equipped in accordance with the proposed invention, their detection capacity would be substantially enhanced. The user then has a choice of using substantially less expensive equipment and material to obtain similar quality data to that presently attainable due to the increased sensitivity of the system, or maintaining the cost of the system and increasing the detection capabilities substantially by usage of this proposed invention.
The invention is a sensor cable system for time domain reflectometry measurement of displacement in a geotechnical material comprising a sensor cable and an anchor attached to the cable, wherein the anchor is configured to be secured within the geotechnical material. Preferably, the sensor cable system comprises a plurality of anchors. Alternatively, the anchor can extend along a substantial portion of the cable and have one or more induced weak points. In embodiments employing a plurality of individual anchors, the anchors are preferably shaped and/or joined with breakable connectors. The induced weak points, shaped anchors and breakable connectors amplify a time domain reflectometry signal caused by displacement of the geotechnical material compared to a signal the displacement would generate in the sensor cable alone. The sensor cable may also comprise a cable motion restrictor, or one or more of the anchors may be configured to comprise a cable motion restrictor. The use of a cable motion restrictor increases the signal generated by displacement of the geotechnical material. The systems are configured to be disposed within a borehole in the geotechnical material and the anchors grouted or otherwise secured within the geotechnical material.
In these systems, the sensor cable may further comprise a TDR signal attenuator. In some embodiments, it may be desirable to configure the anchors so that, when attached to the sensor cable, a TDR response is returned.
The systems of the invention may be configured for use with optical time domain reflectometry (OTDR), electrical time domain reflectometry (TDR) or sound time domain reflectometry (SONAR) techniques and monitors.
The invention also comprises methods of detecting displacement within a geotechnical material which generally comprise the steps of creating a borehole in the geotechnical material, providing a sensor cable system comprising a sensor cable and an anchor attached to the cable, wherein the anchor is configured to be secured within the geotechnical material, disposing the sensor cable within the borehole, depositing grout, or otherwise encapsulating the cable within the borehole to secure the anchor, performing time domain reflectometry to monitor displacement within the geotechnical material.
The proposed system substantially improves detection capability for cable based TDR systems in soils and weak geologic rock formations as well as providing improved performance for cable based TDR systems in hard rock environments due to its unique features and composition.